Metal adhesives containing epoxy resin, polyamide resin and phenolic flux



United States Patent 3,371,008 METAL ADHESIVES CONTAINING EPOXY RESIN, POLYAMIDE RESIN AND PHENOLIC FLUX Eugene F. Lopez, McSpadden Drive, Havre de Grace, Md. 21678 No Drawing. Continuation-impart of application Ser. No. 127,516, July 28, 1961. This application Dec. 17, 1965, Ser. No. 514,634

Claims. (Cl. 161-186) ABSTRACT OF THE DISCLOSURE This invention relates to an adhesive having high metalto-metal bond strength. It is formulated from a polyamide blended with an epoxy resin in which the polyamide is insoluble and with a flux or solvent comprising at least one dior tri-hydroxy phenol.

This application is a continuation-in-part of my copending United States patent application Serial No. 127,- 516 filed July 28, 1961 and now abandoned.

This invention relates to an adhesive. More particularly, it relates to an adhesive having high metal-to-metal bond strengths, formulated from polyamide reacted with epoxy resin.

One recently introduced structural adhesive which has been finding increased acceptance in the aircraft industry is believed to be based on a blend of an epoxy resin with a polyamide soluble therein, the polyamide being one formed from a dicarboxylic acid and diamine. In service, the resulting adhesive produces high metal-to-metal bond strengths at a relatively low modulus, producing equally high bend, fatigue and peel strengths. Unfortunately, in

spite of the advantages possessed by adhesives of the type described, e.g. a decrease in the inherent brittleness of' the epoxy resin and increase in strength of the bond,

these adhesives suffer from one serious disadvantage in that they possess a very poor resistance to water.

One object of this invention is to provide an adhesive yielding high metal-to-metal bond strengths formulated 2 from epoxy resin and a polyamide, and possessing good water resistance.

A more specific object of this invention is to provide metal-to-metal bonded structures wherein the adhesive comprises epoxy resin blended with polyamide and with a solvent which dissolves and fluxes the polyamide with the epoxy resin, and which also reacts with the epoxide and polyamide.

These and other objects are accomplished by utilizing a polyamide which is insoluble in the epoxy resin in combination with a third constituent which acts as a flux and renders the epoxy resin and polyamide compatible. The flux comprises at least one dior tri-hydroxy phenol and preferably a eutectic mixture of two or more polyhydroxy phenols.

3,371,008 Patented Feb. 27, 1968 ICC In the description which follows preferred embodiments of the invention are described by way of illustration and are not to be construed as limitative.

EXAMPLE I LK/ J,

wherein n is preferably about 1.2, (2) 20 parts by weight of nylon 11 (BCl 1107, Belding Corticelli Ind), the nylon being a polymer of ll-amino-undecanoic acid and being insoluble in the mixture of epoxy resins, (3) 6 parts by weight of a fiuxing agent consisting of a 1:1 mixture of resorcinol and catechol, and (4) 3 parts by weight of melamine as a curing agent. The resulting thermosetting composition is then ready to be applied to surfaces to be bonded as the adhesive mixture, whereupon the resulting composite of applied adhesive and elements to be bonded is heated slowly from about room temperature to about 350 F. They are maintained at the temperature of about 350 F. for about 1 hour, which completes the cure and results in a strongly bonded composite structure. During the curing operation the powdered or other solid form of nylon appears to dissolve in the molten phenolic components.

While I do not wish to be bound by any specific theoretical explanation of what then takes place, it appears likely that after the polyamide is taken into the melt, the terminal groups of the polyamide (one carboxylic group and one amino group) react with the epoxide groups causing the molecules of polyamide to become integrally cross-linked into the epoxide structure. During the solvation of the polyamide, the catechol and resorcinol appear to be concurrently dissolving the polyamide and chemically reacting wi h the epoxide. At the same time, the melamine is cross-linking the components.

The resulting adhesive product was tested for sheet strength with the results reported in Table I, below.

EXAMPLE II The procedure of Example I was repeated except that the resorcinol and catechol were omitted from the mixture. The results reported in Table I show the loss in strength in the adhesive, presumably because the insoluble nylon did not react with the epoxy resin.

EXAMPLE III The procedure of Example I was repeated except that the nylon was omitted from the mixture. The results reported in Table I, below, show the loss in strength experienced when the nylon is not present.

9 Table I reports the results of shear tests on /2 inch by 1 inch aluminum lap specimens prepared in accordance with Military Specification MIL-A-509O D.

4 Two modes have been found practical for the practice of the present invention. In either embodiment, the adhesive becomes operative when heated to the melting point of one of the solvents or iluxing agents, or slightly TABLE I o r c 5 h1gher. fif gf {29 3 259 5 In the first mode, illustrated by Example I, the entire (a composition is formulated into a single mixture and ap- 4Y800 31 750 1,000 plied to the surfaces to be bonded, as such. I 2,100 1, 500 500 In the second mode illustrated by the examples which 2700 400 follow, a mixture is formed comprising all of the ingredients except the polyamide, and the resulting com osie s g g i fgg zggz i g i gi moths i non is subsequently applied to a web or scrim of 1nhesivefl g i be com g g i f .2 soluble nylon. The resulting composite is then employed S a S as the bonding material between the elements to be with the Example I resuits. The test is conducted by bonded placing specimens prepared under Military Specification EXAMPLES IVJX MIL-A-5090D in boiling water for 72 hours and then testing immediately at room temperature. A mlXtufe Was formulated y brlnglng together 80 parts by weight of epoxy resin consistmg of parts by TABLE II we1 ht of the to l c1d 1 ether of Example I and 60 parts 3 I 3 y y 3 5?; iigg by weight of the epoxy novolac of Example I, and vanous (p.s.i.) percent amounts of dior trihydroxy phenols as set forth in the 7'000 1,600 77 table wh1ch follows and With about 3 parts by weight of Narmco 405 5,500 1,300 75.5 melamme. Then the compos1t1on was applied to a web Example 4100 or scrim of insoluble nylon. The resulting composite was Obtainable from Bloomingdale Rubber 05. employed as the bonding material between the metals to Narmco Indusmes' be bonded. After heating and curing as in Example I, the Relatively few materials have been found to be suitcomposites were tested for shear strength with the followable 1n the formulation of adhesives from the polyamideing results:

Flux (Pts. by Wt.) Lap Shear Strength (p.s.i.)

Example Resor- Cate- Pyro- Room Boil. cinol ch01 gallol Temp. 180 F. 350 F. Water y) IV 1 3 3, 700 1, 075 550 3,000 3 3 2, 675 2, 425 525 3, 575 6 3 2, 100 1, 300 250 2, 900 9 3 1, 350 1, 550 2, 800 12 3 1, 525 1, 275 2, 450 V 3 1 4,075 2, 250 025 2,100 3 6 2, 075 1, 300 225 2, 575 3 9 2,075 2, 000 125 1, 925 3 12 1,075 1, 300 VI 2 2 4, 065 2, 475 350 3, 400 4 4 ,300 1, 325 125 3, 325 e 0 2, 075 1, 600 2, 700 3 3 1, 325 1,075 700 10 10 325 1, 000 VII 3 1 3,850 2, 900 425 4,000 3 3 3, 025 2, 950 300 4, 100 3 3 3, 875 2, 125 355 4, 300 3 0 4,100 3, 550 350 3, 300 3 12 4, 050 3, 050 300 4,150 VIII 1 3 3, 300 2, 375 325 3, 300 6 3 3, 900 2, 550 350 3, 925 9 3 4, 375 2, 475 150 4,175 12 3 3, 350 1, 375 100 2, 950 IX 2 2 3, 900 2, 450 400 3, 500 4 4 3, 925 2,050 350 3,700 a 0 3,350 2, 925 250 3, 550 3 s 3, 650 2, 725 100 3, 575 10 10 3,700 2,175 100 2, 250

1 No strength.

epoxide resin system described above. Those solvents which have been found effective in dissolving the polyamide and then reacting in the polyamide-epoxide reaction, are solid, non-polymeric phenolic compounds having at least two hydroxyl groups attached to a ring carbon of a benzene ring, and represented by the general formula From Examples IV, V and VI above, it will be seen that when dihydroxyphenols are used as the fluxing agents the lap shear strength falls off as the relative proportions of the constituents departs from the eutectic ratio (1:1 for catechol and resorcinol) and as the amount of fiux exceeds about 9 parts by weight with the parts of epoxy constituent and 20 parts of polyamide.

From Examples VII, VIII and IX it will be apparent that trihydroxyphenol may be substituted for dihydroxyphenol as the flux without adversely affecting the lap shear strength and that when such a change is made the total amount of flux may be somewhat greater than when the flux is entirely dihydroxyphenol.

In general it would appear that the total amount of fiux should comprise from about 2 to up to about 12 parts by weight in a total of parts by weight of epoxy and polyamide.

Having described preferred embodiments of the invention in accordance with the patent statutes, it is not i1- tended that the invention be limited except as required by the appended claims,

wherein R is selected from the group consisting of H, and lower alkyl, and n is an integer which is at least two and not more than three and wherein the proportions of epoxy compound to polyamide are between 3 and 5 parts of epoxy compound for each part of polyamide by weight.

2. The composition of claim 1 wherein the curing agent for said epoxy compound is melamine.

3. The composition of claim 1 wherein the solvent fluxing agent is a mixture of at least two dihydroxyphenols.

4. The composition of claim 1 wherein the solvent and fiuxing agent is a mixture of at least one dihydroxyphenol and at least one trihydroxyphenol.

5. The composition of claim 1 wherein at least a portion of the polyamide is present in the composition as a coarsely woven cloth impregnated with the remaining ingredients of the composition.

6. An adhesive formed by preparing the composition of claim 1 and heating the same to flux and dissolve the polyamide, and maintaining the mixture heated for a time sufiicient for the epoxy compound and polyamide to inter-react.

7. A metal bonded to a metal with a bonding material having the composition of claim 1.

6 8. A composition for adhesively joining metal to metal at temperatures up to about 350 P. which comprises:

(1) about parts by weight of liquid epoxy having an epoxy functionality greater than 2 (2) about 20 parts by weight of poiyamide insoluble in said liquid epoxy, said polyamide being the product of a self-condensation of ll-aminoundecanoic acid (3) between about 3 and 12 parts by weight of a fluxing agent selected from the group consisting of diand trihydroxphenols represented by the general formula wherein R is selected from the group consisting of H and lower alkyl and n is an integer which is at least 2 and not more than 3; and

(4) about 3 parts by weight of an amine curing agent for said epoxy.

9. The composition of claim 8 wherein the curing agent is melamine, and the fiuxing agent is a mixture of resorcinol and catechol.

10. The composition of claim 8 wherein the curing agent is melamine, and the fluxing agent is a 1:1 mixture of resorcinol and catechol.

References Cited Don E. Floyd: Polyamide Resins, 2nd ed., pp. 193-196, Reinhold Publishing Corp, New York 1966.

MORRIS LIEBMAN, Primary Examiner. J. E. CALLAGHAN, Assistant Examiner,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,371,008 February 27, 1968 Eugene F. Lopez It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Columns 3 and 4 in the table ,s'ixth column, line 17 thereof,

for '2,l25" read 3,125 column 6, line 11, for

"trlhydroxphenols" read trihydroxyphenols Signed and sealed this 27th y of y 1969 Edward M. Fletcher, Jr.

Attesting Officer Commissioner of atents 

